This invention relates to methods of, and apparatus for, indicating the direction of a source of magnetic flux.
The invention has particular application in preventing low-flying aircraft, especially helicopters, from colliding with the electrical power transmission lines which run across the country, eg suspended from pylons. These lines are not easy to see, and present a considerable hazard to such aircraft. The present invention provides apparatus which can detect the presence of such lines and indicate at least their approximate direction, alerting the pilot to keep a good look-out in that direction and to take avoiding action.
The invention operates by detecting the low-frequency (50 Hz in the UK) magnetic flux produced by such lines. Transmission lines normally comprise separate cables carrying each of three phases. With perfectly balanced phases (equal currents in each phase), the net magnetic flux at a sufficient distance from the line would be zero, ie at such a distance that the spatially separated cables appeared as a "point" source. It has been found, however, that there is usually sufficient current imbalance between the phases, eg up to 10 or 15%, to produce sufficient net flux to be detectable at a useful distance. Moreover, at distances closer to the line the latter does not appear as a "point" source.
Although the direction of maximum magnetic flux in the plane parallel to the ground and which contains the line is mainly vertical, as may be appreciated by considering the line as a single current-carrying conductor, this vertical component of flux cannot be utilised to give directional information. It is found, however, that owing to the aforesaid imbalance between the currents in the cables, and to the fact that at distances of interest the line is not a "point" source, there is a horizontal component of flux in the aforesaid plane of the transmission line of sufficient strength to enable its direction to be detected. This flux is, however, quite small, eg normally several orders of magnitude less than the "DC" flux due to the earth's magnetic field.